1. Field of the Invention
The invention relates to compatible polymer mixtures comprised of polymethyl methacrylate and styrene-methyl acrylate copolymers.
2. Discussion of the Background
When polymers are mixed there is a high likelihood that they will be incompatible (see Kirk-Othmer, "Encyclopedia of Chemical Technology", Vol. 18, 3rd Ed., pub. John Wiley, pp. 443-478 (1982)). Polystyrene and polyalkyl methacrylates are good examples of incompatible polymers. Thus, M. T. Shaw et al. (see Chem. Abstr. 101:73417e) reported a miscibility of only 3.4 ppm polymethyl methacrylate (PMMA, m.w. 160,000) with polystyrene. Even very low molecular weight polystyrene is relatively incompatible with PMMA (Parent, R. R., et al., J. Polym. Sci., Polym. Phys. Ed., 16:1829 (1987). Other polyalkyl methacrylates are also incompatible with polystyrene. Apparently the only exceptions are mixtures of polystyrene and polycyclohexyl acrylate and polycyclohexyl methacrylate (see Ger. OS 36 32 369). Thus, styrene homopolymer is incompatible with nearly all polyalkyl methacrylates, but this incompatibility does not apply in the case of copolymers of styrene and acrylonitrile. Compatibility has been found between certain styrene/acrylonitrile copolymers and PMMA (see Barlow, J. W. et al., Polymer 28:1177 (1987). However, because this compatibility has apparently been found only for very narrowly specified copolymer compositions of the styrene/acrylonitrile component, the compatibility situation is described as having "miscibility windows". In such cases, the miscibility of the specific styrene/acrylonitrile copolymers with PMMA can be attributed to substantial repulsive forces between the styrene unit and the acrylonitrile unit in the copolymer.
A similar condition is believed to prevail in the case of styrene/maleic anhydride copolymers, which are compatible with PMMA at certain ratios of styrene to maleic anhydride. This group of PMMA-compatible styrene copolymers probably includes copolymers of styrene and allyl alcohol and copolymers of styrene and p-(2-hydroxyhexafluoroisopropyl)styrene, the PMMA-compatibility of which has been interpreted as being due to hydrogen bonding of the hydroxyl group with the PMMA ester group (Min, B. Y., and Pearce, Eli M., Org. Coatings and Plast. Chem., 45:58-64 (1981); Cangelori, F., and Shaw, M. T., Polymer Reprints (Am. Chem. Soc., Div. polym. Chem.), 24:258-259 (1983)).
Although compatibility of PMMA with copolymers of styrene and strongly polar monomers such as acrylonitrile, maleic anhydride, allyl alcohol, and p-(2-hydroxyhexafluoroisopropyl)styrene has been known for a number of years, these compatible polymer mixtures have been and continue to be regarded as special exceptions in the large range of incompatible polystyrene/polyalkyl methacrylate mixtures. This view is particularly understandable in that as a rule the compatibility of these styrene copolymers has been limited to PMMA as a mixture partner.
In certain instances and in certain areas of the plastics industry, mechanical mixtures of polymers have led to products with improved properties, and in some cases the range of possible applications of the subject materials has been substantially broadened (see Kirk-Othmer, loc. cit., Vol. 18). The physical properties of such "polyblends" ordinarily represent a compromise; in favorable cases, such compromises can furnish a net improvement in comparison with the properties of the individual polymers. On the other hand, the technical applicability on incompatible (multiphase) polymer mixtures is often evaluated as better than that of compatible mixtures (see Kirk-Othmer, loc. cit., p. 449).
Incompatible mixtures of PMMA and crosslinked styrene-acrylate ester copolymers have found wide application, with the styrene-acrylate rubber incorporated as a tough (high-impact) phase within a PMMA matrix. Particularly successful have been mixtures of PMMA and copolymers of butyl- or ethyl acrylate with styrene, with styrene present in the amount of about 16 wt. % in the copolymer. Styrenebutyl acrylate copolymers with about 16 wt. % styrene have the same index of refraction as PMMA (n.sup.D.sub.20 =1.492), so that these mixtures are transparent despite being incompatible.
Incompatible mixtures of this type are described, e.g., in Jap. 61/252,263 and 62/164,749, Ger. OS 11 64 080, Jap. 16,818/70, Ger. Pat. App. 33 00 526.5; Hooley, C., et al., Plast. Rubber Process, Appl., 1:345-49, (1981); and Tsutumi, Masahide et al., Jap. 57/39,745. In all these publications the starting material was a styrene acrylate copolymer, as a rule crosslinked, which is incompatible with the PMMA with which it is mixed, and which becomes incorporated in the PMMA matrix. The glass temperature of these styrene-acrylate rubbers is as a rule &lt;0.degree. C. or at least &lt;20.degree. C., and thus these polymers can also act like rubbers. However, a critical factor in their functioning as rubbers is their incompatibility with the PMMA, i.e., the mixed material must be a two-phase material.
Compatible polymer mixtures with favorable mechanical properties can be regarded as technically useful based particularly on the fact that they are expected to have good optical properties. On the other hand, based on the known state of the art, there is little prospect of obtaining technically useful compatible polymer mixtures from components having as their respective components styrene (co)polymers, and polyalkyl (meth)acrylates.